FEATURE: allow admins to enable announced experimental features (#29244)

Toggle the button to enable the experimental site setting from "What's new" announcement.

The toggle button is displayed when:
- site setting exists and is boolean;
- potentially required plugin is enabled.

app/services/experiments/toggle.rb

@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+class Experiments::Toggle
+  include Service::Base
+
+  policy :current_user_is_admin
+
+  contract do
+    attribute :setting_name, :string
+    validates :setting_name, presence: true
+  end
+
+  policy :setting_is_available
+
+  transaction { step :toggle }
+
+  private
+
+  def current_user_is_admin(guardian:)
+    guardian.is_admin?
+  end
+
+  def setting_is_available(contract:)
+    SiteSetting.respond_to?(contract.setting_name)
+  end
+
+  def toggle(contract:, guardian:)
+    SiteSetting.set_and_log(
+      contract.setting_name,
+      !SiteSetting.send(contract.setting_name),
+      guardian.user,
+    )
+  end
+end